Within the field of wind turbines it is necessary to perform work on parts that is situated at a considerable height above ground level (or sea level, when sea wind turbines are concerned) such as e.g. repair of rotor blades, the surface of these, surface treatment of rotor blades and the tower etc. Further, it has been recognized that it is advantageous or even necessary to clean such parts and in particular the rotor blades in order to maintain good results as regards the power efficiency. Further, it may be advantageous to perform other forms of maintenance in order to achieve good power production results and optimal economic results, such as e. g. surface treatments, inspection, etc.
In order to perform such work a number of hoisting arrangements have been proposed in the prior art.
A prior art hoisting arrangement is described in German utility model DE 296 03 278 U. According to this prior art arrangement the wind turbine is brought to a stop with one rotor blade pointing straight downwards, where after suspension means are fastened to each of the two other rotor blades near the hub of the rotor. A special work platform with a through-going slit at the bottom is fixed to these suspension means so that the rotor blade pointing downwards can be inserted into this slit. The work platform is subsequently being hoisted upwards in a stepwise manner, while the crew cleans the surface of the rotor blade manually, e. g. with one person located on each side of the rotor blade.
Even with such an arrangement, it is a time-consuming process to carry out a cleaning of the rotor blades of a windmill, just as such a known arrangement would probably require the use of machinery, such as a crane, for fixation of the special suspension means. Further, the platform itself will have a considerable weight and size, thereby leading to further costs and use of heavy machinery for lifting and lowering the platform.
Even further, the platform is hanging freely from the suspension means, which means that the platform is influenced by the wind and may be moved back and forth, e.g. in windy conditions. Thus, the platform may be a problematic and dangerous working place and it may be impossible to use the platform at even relatively modest wind speeds. Also, even though the platform has rollers at the edge of the slit to lessen damage to the rotor blade, such damages may nevertheless occur in certain conditions.
Devices of a corresponding kind are known from DE 199 09 698 Al and DE 43 39 638 Al, which are encumbered with the same disadvantages as those mentioned above, including that extensive use must be made of special material such as cranes, for example mobile cranes, or relatively comprehensive materials which, for example, are mounted on the turbine tower beforehand. In particular, these prior art arrangements may also be influenced by the wind and may be moved back and forth in relation to the rotor blade, e.g. in windy conditions and when being moved up and down. Thus, these prior art platforms may be problematic working places, it may be impossible to use the platforms at even relatively modest wind speeds, e.g. at relatively high wind turbines, and collisions with the surface of the rotor blade may occur, for example when the platforms are being moved up and down.
The above-mentioned prior art systems are generally not configured in a manner facilitating user-friendliness and do not provide the personnel with an optimal safety environment.
WO 03/048569 A2 describes a method and an apparatus for treatment of a surface of a rotor blade of a windmill, where the apparatus is being placed in such a manner that it is moveable in relation to the surface of a rotor blade, and said apparatus is being caused to move depending on a form of treatment determined by means for treatment mounted on, in or next to the apparatus. In this manner, various forms of treatment of a rotor blade may be carried out such as for instance washing, finishing, sealing etc.
Furthermore, WO 2005/064152 A2 describes a device for enabling access to a structure above ground level by lowering and/or lifting the device in relation to the structure, the device comprising a first endless frame structure defining an opening, wherein at least part of the first endless frame structure forms a track portion, the track portion being adapted to guide an, in relation to the track portion, movable object, such as a gondola, along the track portion.
Both of these prior art documents, i.e. WO 03/048569 A2 and WO 2005/064152 A2 relate to devices, where the devices are transferred to the rotor blade from below the rotor blade tip.
Still further, WO 2004/092577 A1 describes a method of servicing the outer components of a wind turbine such as the wind turbine blades and the tower with a work platform, said method comprising the steps of: positioning the work platform at the wind turbine tower and connecting the work platform to an upper part of the wind turbine with at least one cable. Further the method comprises the steps of raising the work platform with the cable and cable winding means to a position of use, and holding the work platform to the side of the wind turbine tower with holding means.
Even further, WO 2007/085265 A1 discloses a device for enabling access to a structure above ground or sea level, in particular a rotor blade of a wind turbine, by lowering and/or lifting the device in relation to the structure. The device comprises                a frame structure,        means for supporting the device in relation to said structure, and        means for lowering and/or lifting the device in relation to the structure.At least part of the frame structure of this prior art device comprises a track portion, the track portion being adapted to guide an, in relation to the track portion, movable object along the track portion. The means for supporting the device in relation to the structure are configured for positioning and guiding said device in relation to the structure, and the means for supporting the device in relation to said structure are configured for facilitating a movement of said device essentially in the longitudinal direction of said structure        